Shooting the Mangonel
In this dramatic photo the mangonel is caught in the instant before the beam strikes the padding. The massive stone projectile (approx 20kg) in the sling is almost ready to be released.

The mangonel reconstruction is based on Medieval Centre experimentation and the work of Major Schramnn in Germany in the early years of the 20th century - the pioneer of experimental archaeology and the reconstruction of siege engines.

Mangonel Base Timbers
The massive size of the side timbers is particularly visible in this picture.
Late Roman onagers and their medieval equivalents were violent machines that slammed their beams into an upright "catcher". Even with much padding they needed to be very strong to handle this - and to resist the inward crushing of the twisted rope skein that powered them.
On top of all this they tried to kick like wild asses (ie "onagers"), so the heavy base also helped limit their tendency to pitch.

Mangonel violence
The violent nature of mangonels and onagers means that they occasionally break their arms. There are other less obvious aspects too. Here's some additional practical information about the mangonel from Peter Vemming of the Medieval Centre:
"Another thing about the mangonel. First we made the padding that
caught the arm as a stuffed leather cushion, but when we fired the thing,
it got hit with such a force, that the nails holding the cushion flew
like projectiles in all directions. When you use leather the air pressure
gets so big, that it can tear out 2 inch nails. Instead you have to use
canvas, to get rid of the air when the throwing arm hits."

Loading the Mangonel
In this photo the mangonel's arm is being winched back and down against the force of the twisted horsehair rope skein that powers the machine.
This bundle of rope extends from one side of the machine to the other and an end can be seen in the tensioning "washer" near the middle of the side timber.
The power of the machine can be judged by the bending of the bar being used as a handle on the winch.